1. Field of Invention
This invention relates to a vortex flowmeter for measuring the flow rate of a fluid by detecting an alternating signal produced by Karman vortices, wherein a vortex flow rate signal,obtained by an alternating signal passed through a filter and A/D converted, is processed using a microprocessor. More particularly, the invention relates to an improved vortex flowmeter which is characterized by stable flow rate detection.
2. Description of the Prior Art
A vortex flowmeter utilizes the fact that the frequency, at which Karman vortices occur behind a vortex shedder placed in a fluid being measured, is proportional to the flow velocity of the fluid. The vortex flowmeter is widely used to measure flow rates of various fluids because of the flowmeter""s simple structure, wide measurement range, and high measurement accuracy.
FIG. 1 shows an example of a conventional vortex flowmeter, wherein only one sensor is shown, but which can utilize more than one such sensor. An AC charge signal, outputted from a piezoelectric element 1, is converted to an AC voltage signal by a charge converter 2 and the voltage signal is amplified by amplifier 3. The voltage signal amplified by the amplifier 3 carries noise components, in addition to the frequency band being measured. The noise components are removed by a bandpass filter 4. The signal which is passed through the bandpass filter 4 is converted to a pulse signal by a Schmitt trigger 5. The bandpass filter 4 is designed so that the optimum band is selected by means of a microprocessor 7 , which may also be referred to as a central processing unit (CPU),according to the diameter of the vortex flowmeter, fluid density and maximum flow rate.
The signal made into pulses by the Schmitt trigger 5 is introduced to the microprocessor 7, where the pulse signal is subjected to frequency, flow rate and corrective calculations that are performed by calculation means included in the microprocessor 7 using multipliers and calculation programs desired for flow rate calculation. Then, the microprocessor 7 outputs a pulse signal corresponding to the flow rate signal. This pulse signal is converted into an analog signal by a frequency-to-voltage (F/V) converter 8, and then outputted after being changed by a voltage-to-current (V/I) converter 9 into a signal of desired output mode (e.g. a 4-20 mA signal).
The noise components that are superposed on the vortex signal component of the conventional vortex flowmeter include, for example, (a) noise due to the vibration of the piping; (b) low frequency noise such as beat noise; (c) high frequency noise due to, for example, the resonance of the vortex shedder; and (d) spike noise. Although it is possible to significantly reduce the amount of the foregoing noises using a bandpass filter 4, the remaining amount of noise still adversely affects the signal component. Thus, noise components may also be accidently pulsed by the Schmitt trigger 5 into a signal component or the Schmitt trigger 5 may fail to pulse the original signal component.
To solve such problems, it has been suggested in the prior art that following the pulsing of the vortex frequency signal, a judgment be made in an ON-OFF manner using noise judgment means with use of a converted frequency provided by an amplitude-to-frequency (A/F) converter 6, in order to determine whether the component in question is a noise or a signal. However, if the high frequency noise is superposed on a vortex signal, the number of pulses is thus increased the vortex signal is outputted directly as a flow rate signal. This results in reduced accuracy.
Accordingly, an object of the invention is to overcome the aforementioned and other deficiencies and disadvantages of the prior art.
Another object is to provide a vortex flowmeter wherein a low pass filter and means for splitting the frequency band passing through the low pass filter into a plurality of component frequency bands, are located in front of the bandpass filter, the band of the vortex frequency being analyzed according to the signal strength of each band of the component frequency bands, and the band of the bandpass filter being controlled according to the analysis results, whereby flow rate measurement is attained with high precision.